Question: Factor the following expression: $3$ $x^2$ $-5$ $x$ $-28$
This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(3)}{(-28)} &=& -84 \\ {a} + {b} &=& & & {-5} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-84$ and add them together. Remember, since $-84$ is negative, one of the factors must be negative. The factors that add up to ${-5}$ will be your ${a}$ and ${b}$ When ${a}$ is ${7}$ and ${b}$ is ${-12}$ $ \begin{eqnarray} {ab} &=& ({7})({-12}) &=& -84 \\ {a} + {b} &=& {7} + {-12} &=& -5 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {3}x^2 +{7}x {-12}x {-28} $ Group the terms so that there is a common factor in each group: $ ({3}x^2 +{7}x) + ({-12}x {-28}) $ Factor out the common factors: $ x(3x + 7) - 4(3x + 7) $ Notice how $(3x + 7)$ has become a common factor. Factor this out to find the answer. $(3x + 7)(x - 4)$